This study defines the biochemical mechanisms whereby age-dependent changes in hormonal regulation of transport processes perturb physiological control systems and, thus, contribute to the failure to maintain homeostasis in the aged. Topics investigated include: (1) hormonal regulation of phosphate, calcium, salt, and water transport in the kidney; (2) mechanism of action of cholinergic and alpha-adrenergic agonists: phosphoinositol turnover in membranes and calcium flux; and (3) relationships of dopamine receptor levels and dopamine stimulation of adenylate cyclase to dopaminergically mediated rotational behavior.